Hi Wendy - I had the same problem!
This thread on Reddit helped me out - https://www.reddit.com/r/ios/comments/1j2511p/microsoft_exchange_unavailable_to_verify_account/?rdt=63206
Basically, go into Settings / Apps / Safari, then scroll down to Clear History and Data, and then select ALL. Click on the big red button, which will clear everything. I then went into Mail Accounts and re-added my Exchange email account, and it worked. I'm assuming the same might work for passwords too.